Tabular silver halide grains are well known in the art. For example, in Photographic Science and Engineering, Volume 5, No. 6, November-December 1961, Berry, Marino, and Oster study the preparation and growth of silver bromide tabular grains. A discussion of tabular silver bromoiodide emulsions is found in Duffen, Photographic Emulsion Chemistry, Focal Press, 1966, pages 66-72. The patent literature also has dealt extensively with tabular emulsions; for example, Bogg, U.S. Pat. No. 4,063,951 issued Dec. 20, 1977; Lewis, U.S. Pat. No. 4,067,739 issued January, 1978; and Maternaghan, U.S. Pat. Nos. 4,150,994, 4,184,877, and 4,184,878. U.S. Pat. No. 4,439,520, issued Mar. 24, 1984, is directed to high aspect ratio chemically- and spectrally-sensitized tabular grain silver halide emulsions. The emulsions are characterized as having at least 50 percent of the total projected area of the silver halide grains provided by said chemically- and spectrally-sensitized tabular silver halide grains having a thickness of less than 0.3 microns and a diameter of at least 0.6 microns with an average aspect ratio greater than 8.1.
Tabular grains are characterized as having a ratio of diameter to thickness of greater than 1. This ratio is termed "aspect ratio."
U.S. Pat. No. 4,478,929, issued Oct. 23, 1984, is directed to photographic image transfer film units employing a negative working high-aspect ratio tabular grain silver halide emulsion.
It is well known in the art to employ stabilizers to minimize instability in the emulsions either before or after coating, which instability increases fog; that is, the minimum density in the emulsion or in the resulting dye image.
A host of compounds have been employed in the art to stabilize emulsions. One of the most widely-employed class of compounds include the azaindenes as illustrated, for example, in U.S. Pat. Nos. 2,444,607, 2,444,609, 2,449,225, and 2,450,397.
U.S. Pat. Nos. 2,743,180, 2,772,164, 2,835,581, and 3,333,961 are directed to chemically- and optically-sensitive silver halide emulsions employing as anti-foggants or stabilizers specified classes of triazaindenes, tetraazaindenes, and pentaazaindenes.
U.S. Pat. No. 3,161,506 is directed to color diffusion transfer processes which include an optically sensitized silver halide emulsion having a dye developer associated therewith, wherein the emulsion contains a member of the class consisting of hydroxy and amino triazaindenes, hydroxy and amino tetraazeindenes, and hydroxy and amino pentaazaindenes.
Another widely-known and employed stabilizer for emulsions are the mercaptotetrazoles, more specifically 1-phenyl-5-mercaptotetrazole. U.S. Pat. No. 4,332,888, issued June 1, 1982, discloses a method for providing photosensitive silver halide emulsions with enhanced stability employing both a 1-phenyl-5-mercaptotetrazole and a specified azaindene stabilizer.
U.S. Pat. Nos. 4,355,101 and 4,390,613 are directed to specified substituted mercaptotetrazole compounds which are employed as development restrainers in photographic elements and which are particularly useful in diffusion transfer photographic products and processes.
A novel stabilizing system for tabular grains has now been found which is not susceptible to the deficiencies of the prior art.